Bond head assemblies, thermocompression bonding systems and methods of assembling and operating the same

ABSTRACT

A bond head assembly for bonding a semiconductor element to a substrate is provided. The bond head assembly includes a base structure, a heater, and a clamping system securing the heater to the base structure. The clamping system includes a plurality of elastic elements constraining the heater along a plurality of axes.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/042,620, filed Feb. 12, 2016, which claims the benefit of U.S.Provisional Patent Application No. 62/121,868, filed Feb. 27, 2015, thecontents of which is incorporated herein by reference.

FIELD

The invention relates to the formation of electrical interconnections insemiconductor packages, and more particularly, to improvedthermocompression bonding systems and methods of operating the same.

BACKGROUND

In certain aspects of the semiconductor packaging industry,semiconductor elements are bonded to bonding locations. For example, inconventional die attach (also known as die bonding) applications, asemiconductor die is bonded to a bonding location of a substrate (e.g.,a leadframe, another die in stacked die applications, a spacer, etc.).In advanced packaging applications, semiconductor elements (e.g., baresemiconductor die, packaged semiconductor die, etc.) are bonded tobonding locations of a substrate (e.g., a leadframe, a PCB, a carrier, asemiconductor wafer, a BGA substrate, etc.). Conductive structures(e.g., conductive bumps, contact pads, solder bumps, conductive pillars,copper pillars, etc.) provide electrical interconnection between thesemiconductor elements and the bonding locations. In certainapplications these conductive structures may provide electricalinterconnections analogous to wire loops formed using a wire bondingmachine.

In many applications (e.g., thermocompression bonding of semiconductorelements), solder material is included in the conductive structures. Inmany such processes, heat is applied to the semiconductor element beingbonded (e.g., through a heater in a bond head assembly carrying the bondtool). It is important that the application of heat be accomplishedquickly such that the machine throughput (e.g., UPH, or units per hour)is at an acceptable level. This can be challenging as the heater (orparts of the heater) is desirably at different temperatures at differenttimes/locations (e.g., a cooler temperature during removal of thecomponent from a source, such as a wafer, as opposed to a warmertemperature at the time of thermocompressive bonding to a substrate).

Unpredictable expansion and contraction of the heater, resulting fromheating and cooling of the heater, tends to undesirably affect theplacement accuracy in thermocompression bonding applications.

Thus, it would be desirable to provide improved structures and methodsfor thermocompression bonding to desirably maintain the relativeposition of a heater during thermocompression bonding processes.

SUMMARY

According to an exemplary embodiment of the invention, a bond headassembly (also referred to as a “bond head” or “bonding head”) forbonding a semiconductor element to a substrate is provided. The bondhead assembly includes: base structure; a heater; and a clamping systemsecuring the heater to the base structure, the clamping system includinga plurality of elastic elements constraining the heater along aplurality of axes.

Various exemplary details related to such a bond head assembly mayinclude: the heater including a contact portion contacting thesemiconductor element during a bonding process; the bond head assemblyincludes a tool secured to the heater, the tool contacting thesemiconductor element during a bonding process; the heater is formed ofa ceramic material; the elastic elements of the clamping system comprisea material having a coefficient of thermal expansion in a range ofbetween 8-10×10⁻⁶ per degree Celsius, and a thermal conductivity in arange of between 5-10 Watts/(meter×degree Celsius); the base structureincludes an insulating structure having a coefficient of thermalexpansion in a range of between 6-12×10⁻⁶ per degree Celsius, and athermal conductivity in a range of between 1-3 Watts/(meter×degreeCelsius); the base structure defines at least one vacuum channel throughwhich a vacuum is drawn for temporarily securing the semiconductorelement to the heater during a bonding process; the base structuredefines at least one cooling channel configured to transmit a coolingfluid to the heater; wherein the base structure receives electricalcontacts bringing electrical energy to the heater; the plurality ofelastic elements includes a plurality of elements constraining theheater along at least one substantially horizontal axis of the bond headassembly; the plurality of elastic elements includes a plurality ofelements constraining the heater along a z-axis of the bond headassembly; the clamping system includes two clamping structures arrangedon opposite sides of the heater; each of the two clamping structuresincludes ones of the plurality of elastic elements constraining theheater along a plurality of axes of the bond head assembly; each of thetwo clamping structures includes ones of the plurality of elasticelements constraining the heater along a substantially horizontal axisof the bond head assembly and a substantially vertical axis of the bondhead assembly; each of the two clamping structures is formed from aunitary piece of material; another elastic element is secured to each ofthe two clamping structures and constrains the heater along anothersubstantially horizontal axis of the bond head assembly; at least aportion of the plurality of elastic elements are preloaded with theheater; the portion of the plurality of elastic elements are preloadedwith the heater such that the portion of the preloaded plurality ofelastic elements is held in tension; the preloaded portion of theplurality of elastic elements are arranged along a substantiallyhorizontal axis of the bond head assembly; the preloaded portion of theplurality of elastic elements are arranged along a substantiallyvertical axis of the bond head assembly; and the plurality of elasticelements are configured to maintain the heater in a substantiallybalanced state along at least one of the plurality of axes of the bondhead assembly, such that an elastic force acting on the heater issubstantially equal along the at least one axis of the bond headassembly.

According to another exemplary embodiment of the invention, athermocompression bonder is provided. The thermocompression bonderincludes: a semiconductor element supply station including a pluralityof semiconductor elements; a bonding station for holding a substrateconfigured to receive at least one of the semiconductor elements; and abond head assembly for bonding the at least one semiconductor element tothe substrate. The bond head assembly includes a base structure, aheater, and a clamping system for securing the heater to the basestructure. The clamping system includes a plurality of elastic elementsconstraining the heater along a plurality of axes. The bond headassembly of the thermocompression bonder may include any of the variousexemplary details recited in the previous paragraph. Further, thethermocompression bonder may include one or more transfer stations(e.g., see transfer station 170 in FIG. 1).

According to yet another exemplary embodiment of the invention, a methodof assembling a bond head assembly is provided. The method includes thesteps of: securing a heater to a base structure using a clamping system;and constraining the heater along a plurality of axes of the bond headassembly with a plurality of elastic elements of the clamping system.

Various exemplary details related to such a method of assembling a bondhead assembly may include: the heater including a contact portion forcontacting a semiconductor element during a bonding process; a step ofsecuring a tool to the heater wherein the tool is configured to contactthe semiconductor element during a bonding process; the heater beingformed of a ceramic material; the plurality of elastic elements of theclamping system comprising titanium; the plurality of elastic elementsof the clamping system comprising a material having a coefficient ofthermal expansion in a range of between 8-10 (10-6/° C.), and a thermalconductivity in a range of between 5-10 (W/m•° C.); the base structureincluding an insulating structure having a coefficient of thermalexpansion in a range of between 6-12 (10-6/° C.), and a thermalconductivity in a range of between 1-3 (W/m•° C.); the base structuredefining at least one vacuum channel through which a vacuum is drawn fortemporarily securing a semiconductor element to the heater; the basestructure defining at least one cooling channel configured to transmit acooling fluid to the heater; the base structure receiving electricalcontacts for bringing electrical energy to the heater; the heater isconstrained along at least one substantially horizontal axis with onesof the plurality of elastic elements; the heater being constrained alongan x-axis and a y-axis of the bond head assembly with ones of theplurality of elastic elements; the heater being constrained along az-axis of the bond head assembly with ones of the plurality of elasticelements; the step of arranging two clamping structures of the clampingsystem on opposite sides of the heater; the heater being constrainedalong the plurality of axes using ones of the plurality of elasticelements included in each of the two clamping structures; the heaterbeing constrained along a substantially horizontal axis of the bond headassembly and a substantially vertical axis of the bond head assemblyusing ones of the plurality of elastic elements of each of the twoclamping structures; each of the two clamping structures being formedfrom a unitary piece of material; the step of securing another elasticelement to each of the two clamping structures such that the heater isconstrained along another substantially horizontal axis of the bond headassembly; the step of preloading at least a portion of the plurality ofelastic elements using at least one of the base structure and theheater; the step of arranging the preloaded portion of the plurality ofelastic elements along a substantially horizontal axis of the bond headassembly; the step of arranging the preloaded portion of the pluralityof elastic elements along a substantially vertical axis of the bond headassembly; the step of preloading at least a portion of the plurality ofelastic elements with at least one of the base structure and the heatersuch that the preloaded portion of the plurality of elastic elements areheld in tension; and the step of configuring the plurality of elasticelements to maintain the heater in a substantially balanced state alongat least one of the plurality of axes, such that an elastic force actingon the heater is substantially equal along the at least one of theplurality of axes.

According to yet another exemplary embodiment of the invention, a methodof operating a bond head assembly of a thermocompression bonding machineis provided. The method includes the steps of: securing a heater to abase structure using a clamping system, the clamping system including aplurality of elastic elements constraining the heater along a pluralityof axes; and operating the heater in connection with a thermocompressionbonding process.

Various exemplary details related to such a method of operating a bondhead assembly may include: heating the heater, thereby expanding theheater in at least one substantially horizontal axis, and maintainingthe expanded heater in a position relative to the bond head assembly byconstraining the heater along the at least one substantially horizontalaxis using the plurality of elastic elements; the at least onesubstantially horizontal axis including an x-axis and a y-axis of thethermocompression bonding machine; the step of securing the heater tothe base structure including constraining the heater with ones of theelastic elements along the x-axis and the y-axis of thethermocompression bonding machine; expansion of the heater in at leastone of the x-axis and the y-axis results in substantial equalization ofelastic bending in the ones of the elastic elements along the at leastone of the x-axis and the y-axis; a position of a center of the heateralong the x-axis and the y-axis of the thermocompression bonding machineis substantially maintained during operation of the heater; the step ofcooling the heater while maintaining a contact between the heater andthe base structure using the clamping system; the contact between theheater and the base structure is maintained by preloading ones of theplurality of elastic elements along a z-axis of the thermocompressionbonding machine; the heater includes a contact portion for contacting asemiconductor element during the thermocompression bonding process; thestep of securing a tool to the heater wherein the tool is configured tocontact the semiconductor element during the thermocompression bondingprocess; the heater being formed of a ceramic material; the plurality ofelastic elements of the clamping system comprising titanium; theplurality of elastic elements of the clamping system comprise a materialhaving a coefficient of thermal expansion in a range of between 8-10(10-6/° C.), and a thermal conductivity in a range of between 5-10(W/m•° C.); the plurality of elastic elements of the clamping systemcomprising titanium; the plurality of elastic elements of the clampingsystem comprise a material having a coefficient of thermal expansion ina range of between 8-10 (10-6/° C.), and a thermal conductivity in arange of between 5-10 (W/m•° C.); the base structure defining at leastone vacuum channel through which a vacuum is drawn for temporarilysecuring a semiconductor element to the heater; the base structuredefining at least one cooling channel configured to transmit a coolingfluid to the heater; the base structure receiving electrical contactsfor bringing electrical energy to the heater; the heater beingconstrained along an x-axis and a y-axis of the bond head assembly withones of the plurality of elastic elements; the heater being constrainedalong a z-axis of the bond head assembly with ones of the plurality ofelastic elements; the step of arranging two clamping structures of theclamping system on opposite sides of the heater; the heater beingconstrained along the plurality of axes using ones of the plurality ofelastic elements included in each of the two clamping structures; theheater being constrained along a substantially horizontal axis of thebond head assembly and a substantially vertical axis of the bond headassembly using ones of the plurality of elastic elements of each of thetwo clamping structures; each of the two clamping structures is formedfrom a unitary piece of material; the step of securing another elasticelement to each of the two clamping structures such that the heater isconstrained along another substantially horizontal axis of the bond headassembly; the step of preloading at least a portion of the plurality ofelastic elements using at least one of the base structure and theheater; the step of arranging the preloaded portion of the plurality ofelastic elements along a substantially horizontal axis of the bond headassembly; the step of arranging the preloaded portion of the pluralityof elastic elements along a substantially vertical axis of the bond headassembly; the step of preloading at least a portion of the plurality ofelastic elements with at least one of the base structure and the heatersuch that the preloaded portion of the plurality of elastic elements areheld in tension; the step of configuring the plurality of elasticelements to maintain the heater in a substantially balanced state alongat least one of the plurality of axes, such that an elastic force actingon the heater is substantially equal along the at least one of theplurality of axes; and the at least one axis including at least one ofan x-axis and a y-axis of the bond head assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is best understood from the following detailed descriptionwhen read in connection with the accompanying drawings. It is emphasizedthat, according to common practice, the various features of the drawingsare not to scale. On the contrary, the dimensions of the variousfeatures are arbitrarily expanded or reduced for clarity. Included inthe drawings are the following figures:

FIG. 1 is a block diagram view of portions of a thermocompressionbonder, including a bond head assembly, in accordance with an exemplaryembodiment of the invention;

FIGS. 2A-2B are block diagram side sectional views illustrating bondingof a semiconductor element to a substrate using a thermocompressionbonder, including a bond head assembly, in accordance with an exemplaryembodiment of the invention;

FIG. 3A is a block diagram perspective view of a lower bond head of athermocompression bonder in accordance with an exemplary embodiment ofthe invention;

FIGS. 3B-3C are block diagram perspective views of another lower bondhead of a thermocompression bonder in accordance with another exemplaryembodiment of the invention;

FIG. 3D is a block diagram perspective view of a portion of a lower bondhead of a thermocompression bonder illustrating electrical connectionsin accordance with an exemplary embodiment of the invention;

FIGS. 3E-3F are block diagram side sectional views of a portion of alower bond head of a thermocompression bonder illustrating fluid flowpaths in accordance with an exemplary embodiment of the invention;

FIG. 4A-4B are block diagram overhead views of portions of heaterelements of a thermocompression bonder illustrating relative expansiondisplacement of each heater in accordance with an exemplary embodimentof the invention;

FIGS. 5A-5F are block diagram views illustrating elements of a bond headassembly in accordance with an exemplary embodiment of the invention;and

FIGS. 6A-6C are block diagram side views illustrating portions of abonding machine illustrating elastic elements engaged with a heater in arespective cold state, an intermediate heating state and a hot state inaccordance with exemplary embodiments of the invention.

DETAILED DESCRIPTION

As used herein, the term “semiconductor element” is intended to refer toany structure including (or configured to include at a later step) asemiconductor chip or die. Exemplary semiconductor elements include abare semiconductor die, a semiconductor die on a substrate (e.g., aleadframe, a PCB, a carrier, a semiconductor chip, a semicondcutorwafer, a BGA substrate, a semiconductor element, etc.), a packagedsemiconductor device, a flip chip semiconductor device, a die embeddedin a substrate, a stack of semiconductor die, amongst others. Further,the semiconductor element may include an element configured to be bondedor otherwise included in a semiconductor package (e.g., a spacer to bebonded in a stacked die configuration, a substrate, etc.).

As used herein, the terms “substrate” and “workpiece” are intended torefer to any structure to which a semiconductor element may be bonded(e.g., thermocompressively bonded, ultrasonically bonded,thermosonically bonded, die bonded, etc.). Exemplary substrates include,for example, a leadframe, a PCB, a carrier, a semiconductor chip, asemicondcutor wafer, a BGA substrate, a semiconductor element, etc.

Certain exemplary aspects of the invention relate to a bonding head(also referred to as a “bond head” or “bond head assembly”) of a dieattach machine for performing a local reflow solder die attach process.In such a process, the bonding tool places and bonds a semiconductorelement (e.g., a die, an interposer, etc.) to a substrate (e.g., a chip,a wafer, etc.) by melting and re-solidifying solder bumps on thesemiconductor element being placed. This process involves the bondingtool rapidly heating and cooling (e.g., a range of 100s of degreesCelsius, at rates of 100s of degrees per second) while desirablymaintaining a position of the semiconductor element (e.g., to singledigit micron, or smaller, levels).

In accordance with certain aspects of the invention, bonding forces(e.g., forces along the vertical, z-axis) applied to the heater, tool,and semiconductor element being placed/bonded are transferred through arigid insulating structure. In addition the heater is supported in allother orthogonal directions by balanced elastic elements. Since theconstraining elements are desirably balanced, non-symmetric growth willresult in the heater being shifted to a correct center position. Thez-axis clamping element provides forces such that when pressurizedcooling fluid is applied to the non-chip side of the heater the heaterdoes not separate from the supporting structure.

In accordance with certain exemplary aspects of the invention, a designis provided that allows for full support of the heater beingconstrained. Since thermocompression bonding often requires the rapidchange of temperature, it is typically advantageous to reduce thethermal mass of the heater. One such way of reducing thermal mass is tominimize the thickness (while maintaining X,Y dimensions), thus reducingthe volume of material required to be heated, which is done in certainaspects of the invention.

In accordance with certain aspects of the invention, bonding systems(e.g., thermocompression bonding systems) utilizing heat in a bond headassembly (e.g., for melting and/or softening a solder material includedas part of the interconnects of a semiconductor element to be bonded)are disclosed. A bond tool (which may be distinct from the heater, orwhich may be part of the heater) carried by the bond head assemblyplaces and bonds a semiconductor element to a substrate by melting andre-solidifying solder bumps on the semiconductor element beingplaced/bonded. In order to melt the solder bumps, it is desirable torapidly heat the bond tool (via the integrated, or separate, heater)while maintaining the position of the semiconductor element being bonded(e.g., to single digit micron, or smaller, levels). It is also desirableto be able to rapidly cool the bond tool while maintaining the relativeposition of the bonding tool. Thus, it is desirable that bonding systems(and related processes) be capable of precise control of the bond toolpositioning during all phases of the bonding process (e.g., during theheating phase/process, during the cooling phase/process, etc.).

According to various aspects of the invention, the position of theheater, and thus the bonding tool, may be controlled during the heatingphase/process (and the cooling phase/process) of a thermocompressionbonding process. For example, according to certain exemplary embodimentsof the invention, the heater is restrained by clamping elements thatserve to maintain a center of the heater during a heating phase (and acooling phase). This in turn maintains the relative position of abonding tool carried by the heater and any semiconductor elementretained by the bonding tool. A rigid insulating structure carries theheater and insulates the remainder of the bond head assembly from theheating and cooling of the heater.

When the clamping elements first engage the rigid insulating structureand cold heater, the clamping elements are pre-loaded. When the heateris heated, it expands and increases the load on the pre-loaded clampingelements. The clamping elements may be considered as a series of elasticelements constraining the heater along a plurality of axes. As such, anyuneven expansion of the heater as it is heated is compensated for by theseries of elastic elements maintaining/returning to substantiallyneutral positions with equal tension/compression of the elastic elementsas will be described below.

FIG. 1 illustrates exemplary thermocompression bonder 100. Bonder 100includes bond head assembly 100 a including upper bond head 104 (drivenby motion system 102, for example, along the z-axis and the y-axis) andlower bond head 106. Lower bond head 106 is coupled to upper bond head104. As such, various motions of upper bond head 104 will result incorresponding motions of lower bond head 106. Such motions may beprovided, for example, by motion system 102, and/or a rotary motionabout theta axis 150 (with a corresponding motion system not shown), oranother motion system (not shown). Lower bond head 106 includes basestructure 106 a (which desirably includes cooling channels for receivinga fluid for cooling heater 106 b during cooling phases of the process),heater 106 b, and bonding tool 106 c. As will be appreciated by thoseskilled in the art, heater 106 b may be a heated bond tool configured tocarry and bond a semiconductor element (not shown in FIG. 1), and assuch, a separate bonding tool 106 c may be omitted. That is, the termsheater and bond/bonding tool may be used interchangeably, and may beintegrated into a single component (as shown in the exemplary embodimentillustrated in FIG. 3A) or may be multiple separate components (as shownin the exemplary embodiments illustrated in FIG. 1, FIGS. 3B-3C).Heater/bonding tool 106 b/106 c bonds a semiconductor element to asubstrate at bonding station 180. In a direct pick and place embodiment,heater/bonding tool 106 b/106 c may pick a semiconductor element fromsupply station 160 (e.g., a semiconductor wafer or other structureproviding semiconductor elements), and bond the element to a substrateat bonding station 180. In an embodiment using a transfer station (or aplurality of transfer stations if multiple transfers occur), a pick tool(e.g., not shown, but included in transfer station 170) picks asemiconductor element from supply station 160; the semiconductor elementis transferred from the pick tool to heater/bonding tool 106 b/106 c(where such transfer may involve flipping of the semiconductor element),and then the element is bonded to a substrate at bonding station 180.

FIG. 2A illustrates thermocompression bonder 100 including bonding tool106 c (included in lower bond head 106) carrying semiconductor element260 (e.g., a semiconductor die). Upper conductive structures 262 a, 262b (e.g., each including respective conductive pillars 264 a, 264 b suchas copper pillars, and corresponding solder contact portions 266 a, 266b) are provided on semiconductor element 260. Bonding tool 106 c islowered such that such that upper conductive structures 262 a, 262 bcontact lower conductive structures 258 a, 258 b on bonding location 256a of substrate 256 (as will be appreciated, a substrate may include aplurality of bonding locations configured to receive a plurality ofsemicondcutor elements). In the example shown in FIG. 2A, substrate 256is supported by bonding station 180, where bonding station 180 includesa support structure 180 a (e.g., an application specific part) on a bondstage 180 b. In FIG. 2B, through a thermocompressive bonding process,solder contact portions 266 a, 266 b are softened, and thenre-solidified as solder interfaces 266 a 1, 266 b 1, providing apermanent conductive coupling between ones of conductivestructures/pillars 264 a, 264 b and respective lower conductivestructures 258 a, 258 b. Although FIGS. 2A-2B illustrate only two pairsof conductive structures (pair 262 a, 258 a and pair 262 b, 258 b), thisis of course a simple example for ease of explanation. In practice, anynumber of pairs of conductive structures may be provided (e.g., tens ofconductive structure pairs, hundreds of conductive structure pairs,etc.).

FIG. 3A illustrates details of an exemplary lower bond head 106 (such aslower bond head 106 in FIGS. 1 and 2A-2B). Lower bond head 106 is anexample including an integrated heater/bonding tool 106 b/106 c.Heater/bonding tool 106 b/106 c includes a raised portion 106 d (e.g., amesa or protrusion) configured to contact a semiconductor element duringthe thermocompression bonding process. Vacuum hole AA is shown inheater/bonding tool 106 b/106 c through which vacuum may be used tosecure the semiconductor element during movement to the bondinglocation. Lower bond head 106 also includes a holding structure 106 afor holding (constraining) heater/bonding tool 106 b/106 c. Holdingstructure 106 a includes base structure 106 a 1 which may be aninsulative body portion (as such, base structure 106 a 1 may be referredto as insulative body portion 106 a 1). Base structure 106 a 1 definesat least one cooling channel configured to transmit a cooling fluid tothe heater. Holding structure 106 a also includes two sets of clampingstructures 106 a 2 (positioned on opposite sides of insulative bodyportion 106 a 1) for holding heater/bonding tool 106 b/106 c toinsulative body portion 106 a 1. Clamping structures 106 a 2 eachinclude a plurality of elastic elements (e.g., flexures formed fromtitanium or an alloy thereof capable of handling the thermocompressionprocess variables such as temperature changes without losing itsproperties of elasticity, etc.). In the embodiment shown in FIG. 3A,each clamping structure 106 a 2 includes (1) a structure including twoelastic elements 106 a 2′ (side elastic elements) and flexure 106 a 2″(top elastic element) (where all three elastic elements 106 a 2′, 106 a2′ and 106 a 2″ may be formed from a single piece of material), and (2)an elastic backing plate 106 a 2′″. Collectively, the two backing plates106 a 2′″ provide elastic constraint along the x-axis (e.g., as theheater grows and contracts). The two pairs of elastic elements 106 a 2′(4 elastic elements) collectively provide elastic constraint along they-axis (e.g., as the heater grows and contracts). Collectively, the twoelastic elements 106 a 2″ provide elastic constraint along the z-axis(e.g., as the heater grows and contracts). For example, each of the twoelastic elements 106 a 2″ defines a plurality of grooves in the titaniumstructure to provide such elastic constraint along the z-axis.

Through the use of the two clamping structures 106 a 2, elasticconstraint is provided for the heater along each of the x, y, andz-axes, thereby allowing the heater to expand (and contract)substantially centered with respect to its cold (original position).

FIGS. 3B-3C illustrates lower bond head 106′ (where lower bond head 106′may take the place of lower bond head 106 in FIGS. 1 and 2A-2B) in anexample configuration including a heater 106 b′ and a bonding tool 106c′ that are distinct from one another. In FIG. 3B, the bonding tool 106c′ is engaged with heater 106 b′, where a vaccum channel 106 b 1 (e.g.,a lip, a groove, a recess, etc. such as shown in FIG. 3C) is providedwith vacuum from vacuum opening 106 b 2. FIG. 3C illustrates bondingtool 106 c′ removed from heater 106 b′ for illustration. Base structure106 a 1 is similar to base structure 106 a 1 in FIG. 3A, except that itdefines a vacuum channel as in FIG. 3F). Otherwise, the elements ofFIGS. 3B-3C (including the clamping structures 106 a 2) are the same asin FIG. 3A.

FIG. 3D illustrates heater 106 b′ and bonding tool 106 c′ (with theremainder of lower bond head 106′ removed for clarity, but illustratingelectrical contacts/connections 110 (connected to PCB 112) which provideelectricity for heating heater 106 b′ (and to carry other electricalsignals, such as temperature sensor feedback signals).

FIG. 3E illustrates an example with the heater/tool integrated into asingle structure. Base structure 106 a 1 defines a plurality of channelsfor receiving and distributing cooling fluid for cooling heater 106b/106 c (including raised portion 106 d of heater 106 b/106 c) during acooling phase. FIG. 3E illustrates exemplary cooling fluid paths. Itwill be appreciated these the cooling paths may vary significantly fromthose shown. Although the cooling paths are shown as including a portionof heater/tool 106 b/106 c, it is understood that the cooling paths maybe included entirely within base structure 106 a 1, where the cooling isprovided through contact between base structure 106 a 1 and heater/tool106 b/106 c.

FIG. 3F illustrates an example with the heater and tool provided asdistinct structures. As in FIG. 3E, base structure 106 a 1′ defines aplurality of channels for receiving and distributing cooling fluid forcooling heater 106 b′ during a cooling phase. FIG. 3F illustratesexemplary cooling fluid paths. FIG. 3F also illustrates a tool vaccumchannel for holding tool 106 c′ (including raised portion 106 d) toheater 106 b′ using vacuum. Although the cooling paths are shown asincluding a portion of heater 106 b′, it is understood that the coolingpaths may be included entirely within base structure 106 a 1′, where thecooling is provided through contact between base structure 106 a 1′ andheater 106 b′.

FIG. 4A is an overhead block diagram view of parts of athermocompression bonder including a heater in a cold state (400 a) andan expanded hot state (400 b), where the teachings of FIGS. 4A-4B may beapplied to any heater (e.g., heater/tool 106 b/106 c, heater 106 b′,etc.) in accordance with the invention. The center points “C” of thecold heater 400 a and the hot heater 400 b coincide, and thus FIG. 4Aillustrates an idealized heater expansion with no motion of the centerpoint. Thus, stress exerted at each of the four corners of the coldheater 400 a (illustrated as 4 arrows in FIG. 4A) while expanding duringthe heating process are substantially equal. However, and as illustratedin FIG. 4B, when cold heater 400 a is heated, it may expand unevenly sothat center point “C2” of the hot heater 400 b shifts away from centerpoint “C1” of the cold heater 400 a. Thus, stress exerted at each cornerof the cold heater 400 a while expanding during the heating process areunequal. This uneven expansion of the hot heater 400 b also shifts abonding tool carried by hot heater 400 b (or integrated with the heater)and thus any semiconductor element held by the tool. This results in anunacceptable movement of the semiconductor element in relation to a workpiece to which the semiconductor element may be bonded, and may resultin a fatal defect in the bonded semiconductor assembly.

FIG. 5A illustrates block diagram portions of thermocompression bonder100 (e.g., a thermocompression flip chip bonding machine), such asincluding lower bond head 106′ shown in FIGS. 3B-3C, including clampingstructures 106 a 2 (including the various clamping elements) on eachside of a rigid insulating structure 106 a 1′ and heater 106 b′. Heater106 b′ is cold and carries (cold) bond tool 106 c′, which in turn maycarry a semiconductor element (not shown) for bonding to a substrate(not shown). FIG. 5C illustrates a clamping structure 106 a 2 in anunclamped, unloaded state, before engaging/clamping rigid insulatingstructure 106 a 1 and heater 106 b′. One end/portion “AA” of eachclamping element structure 106 a 2 is carried by a fixed surface (e.g.,a portion of the bond head assembly), and is separated from an opposingend “CC” of the clamping structure 106 a 2 by elastic portion “BB”represented in FIG. 5C as a spring structure. The opposing end “CC” ofeach clamping structure 106 a 2 has an L-shape configured to engage alower end of heater 106 b′. When the clamping structures 106 a 2 engagethe insulating structure 106 a 1 and the cold heater 106 b′ (in apreloaded state, as in FIG. 5D), elastic portion “BB” stretches totension/load the clamping structures 106 a 2 as indicated by the doubleheaded arrow in FIG. 5D (see BB′) so that the elastic portion is undertension. The loading of the clamping structures 106 a 2 creates arelatively large force.

FIG. 5B illustrates heater 106 b′ having been heated to expand in X, Yand Z directions (see the legends). This expansion of heater 106 b′ isopposed by the loaded clamping system (including the pair of clampingstructures 106 a 2), and creates further loaded clamping elements. Theincrease in force generated by the heating of heater 106 b′ is less thanthe preload force (in FIG. 5D). In FIG. 5D, the preload displacement(along the z-axis) is large compared to additional z-axis displacementin due to heating in FIG. 5E. The construction of clamping system(including the clamping structures 106 a 2) substantially maintains thecenter of hot heater 106 b′ (carrying the heated tool) relative to thecenter of the cold heater 106 b′.

FIG. 5F is a bottom view of FIG. 5A (with the tool removed). Pre-loadedclamping structures 106 a 2 extend along opposing sides of the coldheater 106 b′, where backing plates 106 a 2′″ (not shown in FIGS. 5A-5Efor simplicity, but see FIGS. 3A-3C) overlie the opposing outer sides ofthe loaded clamping structures 106 a 2. Backing plates 106 a 2′″ serveto provide elastic constraint along the x-axis. The loaded clampingstructures 106 a 2 include projections to engage and retain the heater106 b′ along a bottom surface (with the upper surface of the heater 106b′ being against a fixed surface, such as a part of the bond headassembly).

As will be appreciated by those skilled in the art, the teachings ofFIGS. 5A-5F may be applied to any embodiment of the invention. Forexample, clamping structures 106 a 2 may be the same as shown in theother drawings, etc.

FIGS. 6A-6C illustrate top down block diagram views of a cold heater 600(FIG. 6A), the heating of cold heater 600 (FIG. 6B), and a hot heater600 (FIG. 6C), where the teachings of FIGS. 6A-6C may be applied to anyheater (e.g., heater/tool 106 b/106 c, heater 106 b′, etc.) inaccordance with the invention. The elastic members of the clampingstructures/elements/backplates are simplified as spring member pairsKy1, Ky3; Ky2, Ky4/Kx1, Kx3; and Kx2, Kx4; however, it is understoodthat the other clamping systems (e.g., such as the previouslyillustrated and described clamping structures 106 a 2 may provide thefunction of the spring member pairs shown in FIGS. 6A-6C). FIG. 6Aillustrates cold heater 600 with equal tension/compression of springmember pairs Ky1, Ky3; Ky2, Ky4/Kx1, Kx3; Kx2, Kx4. FIG. 6B illustratesthe heater 600 during the process of being heated. As illustrated, theheater 600 expands to the right in relation to FIGS. 6A-6C towardssprings Kx3, Kx4: compressing springs Kx3, Kx4; elongating springs Kx1,Kx2; and bending springs Ky1, Ky2; Ky3, Ky4 towards the right. Elasticmembers/springs tend to balance out the tension/compression/bending inall elastic members/springs. As illustrated in FIG. 6C, center “C” ofthe unequally expanded hot heater 600 returns (may be maintained) to thesame relative position as center “C” of the cold heater 600 (in FIG.6A). This ensures a bonding tool carried by the heater 600 (orintegrated with the heater), and any semiconductor element retained bybonding tool, remain substantially centered.

Although the invention provides specific examples of clampingsystems/structures (and associated elastic elements) for elasticallyconstraining the heater along certain axes, it is understood that theseexamples are non-limiting. That is, various changes may be made to thestructure of the clamping structures (including how elastic constraintis provided along each of the x, y, and z-axes) within the scope of theinvention. As a specific example, the backing plate provided for elasticconstraint along the x-axis could be done in other ways without abacking plate, for example, through further spring function integratedinto the clamping elements.

Although certain aspects of the invention have been illustrated withcertain motion axes, it is understood that these are exemplary innature.

Although the invention is illustrated and described herein withreference to specific embodiments, the invention is not intended to belimited to the details shown. Rather, various modifications may be madein the details within the scope and range of equivalents of the claimsand without departing from the invention.

What is claimed:
 1. A bond head assembly for bonding a semiconductorelement to a substrate, the bond head assembly comprising: a basestructure; a heater; and a clamping system securing the heater to thebase structure, the clamping system including a plurality of elasticelements constraining the heater along a plurality of axes, theplurality of elastic elements maintaining a center of the heater withrespect to at least one of the plurality of axes during (i) a heatingphase of a thermocompression bonding process and (ii) a cooling phase ofthe thermocompression bonding process.
 2. The bond head assembly ofclaim 1 wherein the heater includes a contact portion contacting thesemiconductor element during a bonding process.
 3. The bond headassembly of claim 1 wherein the bond head assembly includes a toolsecured to the heater, the tool contacting the semiconductor elementduring a bonding process.
 4. The bond head assembly of claim 1 whereinthe heater is formed of a ceramic material.
 5. The bond head assembly ofclaim 1 wherein the elastic elements of the clamping system comprisetitanium.
 6. The bond head assembly of claim 1 wherein the elasticelements of the clamping system comprise a material having a coefficientof thermal expansion in a range of between 8-10×10⁻⁶ per degree Celsius,and a thermal conductivity in a range of between 5-10Watts/(meter×degree Celsius).
 7. The bond head assembly of claim 1wherein the base structure includes an insulating structure having acoefficient of thermal expansion in a range of between 6-12×10⁻⁶ perdegree Celsius, and a thermal conductivity in a range of between 1-3Watts/(meter×degree Celsius).
 8. The bond head assembly of claim 1wherein the base structure defines at least one vacuum channel throughwhich a vacuum is drawn for temporarily securing the semiconductorelement to the heater during a bonding process.
 9. The bond headassembly of claim 1 wherein the base structure defines at least onecooling channel configured to transmit a cooling fluid to the heater.10. The bond head assembly of claim 1 wherein the base structurereceives electrical contacts bringing electrical energy to the heater.11. The bond head assembly of claim 1 wherein the plurality of elasticelements includes a plurality of elements constraining the heater alongat least one substantially horizontal axis of the bond head assembly.12. The bond head assembly of claim 1 wherein the plurality of elasticelements includes a plurality of elements constraining the heater alongan x-axis of the bond head assembly and a y-axis of the bond headassembly.
 13. The bond head assembly of claim 1 wherein the plurality ofelastic elements includes a plurality of elements constraining theheater along a z-axis of the bond head assembly.
 14. The bond headassembly of claim 1 wherein the clamping system includes two clampingstructures arranged on opposite sides of the heater.
 15. The bond headassembly of claim 14 wherein each of the two clamping structuresincludes ones of the plurality of elastic elements constraining theheater along a plurality of axes of the bond head assembly.
 16. The bondhead assembly of claim 14 wherein each of the two clamping structuresincludes ones of the plurality of elastic elements constraining theheater along a substantially horizontal axis of the bond head assemblyand a substantially vertical axis of the bond head assembly.
 17. Thebond head assembly of claim 16 wherein each of the two clampingstructures is formed from a unitary piece of material.
 18. The bond headassembly of claim 16 wherein another elastic element is secured to eachof the two clamping structures and constrains the heater along anothersubstantially horizontal axis of the bond head assembly.
 19. The bondhead assembly of claim 1 wherein at least a portion of the plurality ofelastic elements are preloaded with the heater.
 20. The bond headassembly of claim 19 wherein the portion of the plurality of elasticelements are preloaded with the heater such that the portion of thepreloaded plurality of elastic elements is held in tension.
 21. The bondhead assembly of claim 19 wherein the preloaded portion of the pluralityof elastic elements are arranged along a substantially horizontal axisof the bond head assembly.
 22. The bond head assembly of claim 19wherein the preloaded portion of the plurality of elastic elements arearranged along a substantially vertical axis of the bond head assembly.23. The bond head assembly of claim 1 wherein the at least one axisincludes at least one of an x-axis of the bond head assembly and ay-axis of the bond head assembly.
 24. A thermocompression bondercomprising: a semiconductor element supply station including a pluralityof semiconductor elements; a bonding station for holding a substrateconfigured to receive at least one of the semiconductor elements; and abond head assembly for bonding the at least one semiconductor element tothe substrate, the bond head assembly including a base structure, aheater, and a clamping system for securing the heater to the basestructure, the clamping system including a plurality of elastic elementsconstraining the heater along a plurality of axes, the plurality ofelastic elements maintaining a center of the heater with respect to atleast one of the plurality of axes during (i) a heating phase of athermocompression bonding process and (ii) a cooling phase of thethermocompression bonding process.
 25. A method of assembling a bondhead assembly, the method comprising the steps of: securing a heater toa base structure using a clamping system; constraining the heater alonga plurality of axes of the bond head assembly with a plurality ofelastic elements of the clamping system; and using the plurality ofelastic elements maintain a center of the heater with respect to atleast one of the plurality of axes during (i) a heating phase of athermocompression bonding process and (ii) a cooling phase of thethermocompression bonding process.
 26. A method of operating a bond headassembly of a thermocompression bonding machine, the method comprisingthe steps of: securing a heater to a base structure using a clampingsystem, the clamping system including a plurality of elastic elementsconstraining the heater along a plurality of axes; operating the heaterin connection with a thermocompression bonding process; and using theplurality of elastic elements maintain a center of the heater withrespect to at least one of the plurality of axes during (i) a heatingphase of a thermocompression bonding process and (ii) a cooling phase ofthe thermocompression bonding process.